"Black Monday" in 1929 refers to October 28, 1929, a day of panic and a massive drop in the stock market, with the Dow Jones Industrial Average falling by 12.8%. This was the first of two days of extreme decline; it was followed by "Black Tuesday," October 29, when the market dropped an additional 12% and over 16 million shares were traded. The two days of sharp declines are seen as the beginning of the 1929 stock market crash, a major event that helped trigger the Great Depression. Mass panic: The sell-off was fueled by panic selling, as investors rushed to get out of the market and more investors faced margin calls. Record losses: On October 28, the Dow lost 12.8% of its value, the largest one-day percentage drop at the time. Continued decline: The panic continued on Black Tuesday, with the market dropping another 12% and a record 16 million shares being traded. Financial implications: By mid-November, the Dow had lost almost half of its value, and the crash played a significant role in the economic collapse that led to the Great Depression.